The mechanical response of isolated arteries to potassium

Skeletal muscle beta 2-adrenoreceptors and the effect of adrenergic drugs on plasma potassium in perinephritis hypertension in rabbits

1. It has been suggested that beta 2-adrenoreceptors in skeletal muscle regulate plasma potassium. The possibility that alterations in the function and/or density of these receptors occurs in perinephritis hypertension in rabbits was studied. 2. Intravenous infusion of adrenaline (0.2 micrograms kg-1 min-1) caused a fall in potassium while intravenous bolus injection of propranolol (0.75 mg kg-1) resulted in an increase in serum potassium which was of similar magnitude in both perinephritis hypertensive and sham-operated normotensive rabbits. 3. Binding studies with the radioligand [125I] cyanopindolol (ICYP) showed that there were no significant differences between the hypertensive and normotensive rabbits in the density (Bmax) or affinity (KD) of the skeletal muscle beta 2-adrenoreceptor. 4. The results suggest that function and density of skeletal muscle beta 2-adrenoreceptors are not altered in rabbits with perinephritis hypertension.

Extracellular potassium ion dynamics and ventricular arrhythmias in the canine heart

The relation between extracellular potassium ion activity [( K+]o) and ventricular tachyarrhythmias was studied in an open chest canine model with the use of two protocols. In Protocol I, potassium chloride was administered into the proximal left anterior descending coronary artery at a rate of 0.125 mEq/min for either 20 min or until [K+]o = 20 mEq/liter, whichever came first. In Protocol II, the proximal left anterior descending coronary artery was occluded in one step and was reperfused 20 min later. Fifteen dogs were subjected to Protocol I, nine of which were also subjected to Protocol II. In the latter group, a recovery period of greater than or equal to 1 h separated the two protocols. Local K+ and intramyocardial activities were recorded with use of bifunctional ion-sensitive plunge electrodes at multiple sites located in the region of the left ventricle perfused by the left anterior descending artery and at one site outside of this region. The following variables were recorded and analyzed: Lead II electrocardiogram, heart rate, systemic arterial blood pressure, local [K+]o and its time derivative (dK+/dt), local electrograms and ventricular arrhythmias. Maximal [K+]o and dK+/dt were 23 +/- 3 mEq/liter and 9 +/- 1 mEq/liter per min in Protocol I and 14 +/- 1 mEq/liter and 3 +/- 1 mEq/liter per min in Protocol II, respectively. In both protocols, the occurrence of ventricular arrhythmias correlated with [K+]o (p less than 0.02) as well as with dK+/dt (p less than 0.05). Ventricular arrhythmias were more frequent and more severe in Protocol II than in Protocol I (p less than 0.05). Therefore, whereas K+ dynamics were more pronounced in Protocol I, ventricular arrhythmias were more severe in Protocol II. This occurrence was apparently due, at least in part, to less heterogeneous changes in K+ gradients during constant K+ infusion. It was concluded that, in addition to the magnitude of [K+]o, the rate of change of this variable (that is, dK+/dt) apparently plays an important role in the genesis of ischemic ventricular arrhythmias.

Potassium and norepinephrine- or angiotensin-mediated pressor control in pre-hypertension

Blood pressure (BP), plasma electrolytes, renin, aldosterone, angiotensin II (AII) or catecholamines, the chronotropic effects of intravenous isoproterenol, norepinephrine (NE) or AII, the pressor responses to NE or AII, and the relationship between plasma AII and aldosterone concentrations were studied before and after 10 days of dietary supplementation with potassium 100 mmol/day, in normotensive members of normotensive (N = 12) or hypertensive (N = 12) families, and 11 patients with borderline essential hypertension. Under control conditions, the pressor responsiveness to NE was significantly enhanced in normotensive with positive family history for hypertension and hypertensive subjects; the other variables were comparable in the groups. After potassium supplementation, plasma potassium, renin, aldosterone or AII, and the relationship between AII and aldosterone levels increased significantly, while body weight, plasma catecholamines, the chronotropic effects of isoproterenol, AII or NE, the pressor effects of AII and plasma clearance of AII or NE were unchanged in all groups. In normotensive members of hypertensive families and patients with hypertension, BP was decreased and the exaggerated pressor responsiveness to NE was normalized; these variables were not modified in normotensive members of normotensive families. These observations are consistent with a potassium-remediable disturbance in NE- but not AII-dependent regulation of BP in the pathogenesis of essential hypertension.

Blood cardioplegia delivery. Deleterious effects of potassium versus lidocaine

Delivery of cardioplegic (CP) solutions to all regions of the myocardium is critical for optimal myocardial protection during cardiac surgery. However, there are little data regarding the effects of CP agents upon coronary vascular resistance (CVR) and CP delivery. Accordingly, we evaluated blood CP (Hct 30) delivery and CVR during 75 minutes of multi-dose hypothermic blood CP arrest in an in vivo isolated dog heart preparation. Three groups of dogs were studied: K(K+ = 30 mEq/L; n = 6), L (Lidocaine = 400 mg/L; K+ = 4 mEq/L; n = 6), and KL (K+ = 30 mEq/L, Lidocaine 400 mg/L; n = 6) during total cardiopulmonary bypass and moderate systemic hypothermia (28 C). Basal CVR was calculated by measuring total coronary flow (HR 120/min; mean aortic pressure = 80 mmHg) in the empty beating heart. After aortic cross-clamping, the blood CP solution was infused into the aortic root at a constant pressure (80 mmHg) and constant temperature (16 +/- 2 C) for 60 seconds at 15 minute intervals for a total arrest time of 75 min. Total CP flow, CVR, O2 consumption, lactate extraction/production, and K+ balance during 75 minutes of arrest and 30 minutes of reperfusion were determined. The distribution of the CP solution in the left ventricle was measured with radioactive microspheres (9 +/- 1 mu). Biopsy specimens were taken to measure wet to dry ratios. Values are mean +/- SEM. Data were analyzed by BMDP-P2V. During the first CP infusion, after aortic cross-clamping, no differences in CVR or CP distribution were found among the three groups. However, CVR was increased significantly in the K group during the second CP infusion (O': 0.98 +/- 0.20 mmHg/ml/min/100 g; 15': 2.66 +/- 0.82; p less than 0.001). The CVR remained high for the remainder of the arrest period. Moreover, total, epi- and endocardial flow decreased significantly (54%, p less than 0.001). In groups L and KL, no significant changes in CVR were seen. Groups K and KL showed a significant K+ extraction during the first CP infusion. During the early reperfusion period, K+ washout occurred in these two groups, which was not seen in the L group. There was no significant difference between the three groups in myocardial O2 consumption, lactate metabolism, and water content during the arrest and the reperfusion period. In conclusion, high concentrations of K+ (30 mEq/L) can markedly increase CVR and impair blood CP delivery and distribution. These effects can be prevented by lidocaine. These findings warrant reassessment of the various additives to CP solutions and their effects on CVR and CP distribution during multi-dose hypothermic CP arrest.

The role of potassium in the metabolic control of coronary vascular resistance of the dog

We tested the hypothesis that potassium ion (K+) is involved in the local control of the coronary circulation. The left coronary artery was perfused at constant flow in closed-chest, anesthetized dogs. Step increases in heart rate caused transient (six dogs) or sustained (three dogs) increases in coronary sinus plasma [K+] averaging 0.53 mEq/liter. When the effects of vascular transit delay were accounted for, we found that [K+] changes preceded the vasodilation seen with increased heart rate. We used a mathematical model to calculate changes in interstitial [K+] from arterial and venous [K+] and K+ release rate. The magnitude of the changes in interstitial [K+] appeared to be sufficient to account for a considerable portion but not all of the initial changes in coronary vascular resistance associated with increased heart rate. Thus potassium seems to be involved at least transiently, and, in three of nine dogs, for a more sustained period, in heart rate-induced coronary vasodilation. Cessations (for 15 seconds) of coronary blood flow resulted in transient postischemic increases of coronary sinus [K+] averaging 0.55 mEq/liter. In this case, correction for vascular transit disclosed that the recovery of [K+] preceded the return of vascular tone to baseline for only the second half of the recovery, implying only a limited role for potassium in this response. Potassium appears to play a significant but transient role in the local control of the coronary circulation.

Action and interaction of perivascular H+, K+ and Ca++ on pial arteries

In cats the perivascular space of small pial arteries was perfused with mock CSF by means of micropipettes. Variations of K+, H+ and Ca++ activities in the perfusate cause changes of the diameters of the affected vessels. Increases of H+ and moderate increases of K+ lead to dilatations, Ca++ -increases elicit constrictions. The ions interact in such a way that perivascular Ca++ inhibits the K+ or H+ induced dilator response of the pial arteries. Alkalosis intensifies the constrictor action of Ca++. Perivascular microperfusion with Ca++ -free mock CSF causes dilatation which can be augmented by additional acidosis (pH 6.5). However, an increase of K+ to 10.8-10(-3) mol.1(-1) (4 times normal) does not change the diameters of the vessels already dilated, thus indicating differences in the action of K+ and H+ upon the membranes of the pial vascular smooth muscle cells. Small vessels (less than 80 micron diameter) respond with a significantly stronger dilatation when Ca++ is complexed with EGTA as compared with vessels of more than 80 micron diameter.

The mechanism of K+-induced vasodilation of the coronary vascular bed of the dog

We tested a number of hypotheses concerning the mechanism of K+-induced vasodilation of the coronary vascular bed. Blood flow in the circumflex artery was measured in pentobarbital-anesthetized, open-chest dogs. Intracircumflex artery bolus injections of 40 mumol of isomotic KCl produced decreases in coronary vascular resistance ranging from 34% to 48%, depending on the initial resistance of the vascular bed. K+ administration had no effect on heart rate and produced a 4 mm Hg decrease in mean arterial pressure. K+ injection caused a 0.2 vol% increase in coronary sinus O2 content in a preparation in which left common coronary flow was held constant. The magnitude of K+-induced vasodilation was not significantly affected by the administration of propranolol, atropine, phentolamine, or lidocaine. K+-induced vasodilation was attenuated (50%) by ouabain plus lidocaine. Acetylcholine-induced vasodilation was not significantly diminished by ouabain plus lidocaine. We conclude that the mechanism of K+-induced vasodilation does not involve an increase in the metabolic activity of the heart or an interaction between K+ and tissue neural elements. Our data do support the hypothesis that K+-induced vasodilation is at least partly the result of an activation of the electrogenic Na+-K+ transport system of coronary smooth muscle.

Evidence in support of hypoxia but against high potassium and hyperosmolarity as possible mediators of sustained vasodilation in rabbit cardiac and skeletal muscle

The vasoactive properties of hypoxia, elevated extracellular potassium concentration ([K] o ), and hyperosmolarity were studied in helical strip preparations of small coronary and deep femoral arteries (260-700 µm, o.d.) equilibrated in a physiological salt solution with an oxygen tension (Po 2 ) of 100 mm Hg, [K] o of 3.18 mM, and an osmolarity of 304 milliosmoles/liter. Increasing [K] o (2-6 mM) or osmolarity (30-50 milliosmoles/liter) produced relaxation of resting tension in 50% of the coronary strips but had no effect on resting tension in deep femoral strips. Sustained increments in [K] o or osmolarity produced concentration-dependent, transient relaxation of aganist-induced contractile tension in both coronary and deep femoral arterial strips: a potassium increment of 4 mM produced 40% relaxation with 100% recovery within 5-6 minutes, an osmotic increment of 30 milliosmoles/liter caused 20-40% relaxation with 100% recovery within 15-60 minutes, and simultaneous potassium (4 mM) and osmotic (30 milliosmoles/liter) increments produced 85-95% relaxation with 100% recovery within 10-15 minutes. Decreasing Po 2 from 100 mm Hg to 10 mm Hg resulted in a sustained 35-40% fall in agonist-induced contractile tension. Although a nonspecific additive interaction was observed during a simultaneous change to high [K] o , hyperosmolarity, and hypoxia, for any given level of vascular tone hypoxia had little or no effect on the degree or the duration of the tension relaxation-recovery sequence produced by elevated [K] o , hyperosmolarity, or both. Therefore, it is proposed that hypoxia is the only one of these three factors that, by a direct interaction with vascular smooth muscle cells, can contribute to sustained vasodilation of small arteries in rabbit cardiac or skeletal muscle.